Constellation Operations Challenges

4.3.1 Spacecraft Bus GPS Receiver Non-Fixed Issue

The spacecraft bus GPSR of FM1, FM3, FM4, and FM6 could not reliably acquire and lock onto the signals from the GPS constellation, as shown in Figure 4-4 and 4-5. The bus GPSR sometimes provides erroneous data, causing problems in the TIP payload time stamping, ACS navigation processing, and the onboard timing system. These data problems

cause the navigation to output erroneous data and result in erratic attitude excursions behaviors on the spacecraft. The issue has been resolved by inhibiting any state vector solution from the bus GPSR and then commanding four known state vectors daily to each corresponding spacecraft from SOCC. The state vector is obtained from the GOX payload.

FM5 and FM2 were chosen as the first two spacecraft to be raised from their parking orbit, since their bus GPSRs were behaving nominally. This allowed the team to perform orbit determination using the data from the spacecraft bus GPSR. As for the other four spacecraft (FM1, FM3, FM4 and FM6), NSPO has modified the thrusting procedure to include GOX operations as part of burn activities [31]-[32], [61].

4.3.2 High Beta Angle Effect

There were thermal anomalies related to orbital high beta angles. At high beta angles, the spacecraft were in constant sunlight. This causes the earth horizon sensor (EHS) temperature to become higher than expected. Additionally the battery pressures rose higher and closer to the specified limit during this time period. To solve this issue, TIP and TBB were turned off when the beta angle was higher than 60^o. To resolve the battery pressure issue, the charge rate was fine-tuned to maintain the battery within the normal pressure limit through frequent monitoring and commanding. The power control flight software was subsequently modified to include a new battery overpressure protection function and this was successfully uploaded early in 2007. Currently the battery pressure is being maintained at nominal condition autonomously [31]-[32], [61].

4.3.3 Spacecraft Computers Reset/Reboot

A total of 87 out of 102 recorded spacecraft resets and reboots events including Flight Computer (FC), Battery Charge Regulator (BCR) and Attitude Control Electronics (ACE) have been observed after two years in orbit since launch. Figure 4-6 shows the projected geographic locations of these reset/reboot events on the Earth after two years in orbit.. .

Further investigation shows that most of the time and geo-locations the spacecraft anomalies occurred are closely correlated to the space radiation environment. Single Event Effects in the South Atlantic Anomaly (SAA) region and the polar region are identified as the most probable root cause. The spacecraft will recovers from system level Failure Detection &

Correction (FDC) strategies after resets and reboots events occurred, and no spacecraft performance has been degraded by these anomalies [31]-[32], [61].

4.3.4 Spacecraft FM2 Power Shortage

As shown in Figure 4-7, generally the average solar power falls into 140~150W with a 200W solar array power capacity in design. Actual flight experience shows that battery capacity is greater than specified value in typical normal operation. The maximum battery capacity or SOC can be as high as 15Ah after being charged. The peak power-tracking scheme can maintain the solar array at its maximum power output, but it is restricted by maximum battery charge current as well. On March 1, 2007 the operations team observed that the maximum power capacity of the solar arrays had been reduced from 200 W to 100 W by about 50%. FM2 had experienced a sudden solar array power shortage. The effect was deemed to be mechanical and resulted in a permanent power failure from one solar array. An investigation of this power shortage anomaly resulted in a recovery plan to operate the GOX at a reduced-duty cycle. Currently FM2 is supporting the GOX at ~70% duty cycles with the secondary payloads remaining off at all times [31]-[32], [61].

4.3.5 Spacecraft FM3 Solar Array Lockout

On August 3, 2007 FM3 encountered the solar array drive mechanism malfunction when it reached a 711 km orbit. The stuck solar array effects were two-folded, one was to block the thrusting to continue to 800 km mission orbit, and the other one was the lost sun tracking capability of solar array for the spacecraft. Currently FM3 is able to operate the GOX at a

~50% duty cycle with TBB and TIP payloads turned off at all times. The reasons for this

anomaly are still under investigation [32], [61].

4.3.6 Spacecraft FM6 67-Days Outage

Spacecraft flight model number 6 (FM6) lost its communication on 8 Sep. 2007 [65].

There was no warning that indicated a spacecraft problem prior to the FM6 outage event.

Many emergency recovery attempts were tried by the operations team, without success.

However, after 67 days the FM6 resumed contact and recovered back on its own after a computer master reset event occurred over the SAA region. The FM6 transmitter’s RF spectrum looked normal with no sign of degradation and all the spacecraft subsystems were found to be in good health status. The FM6 started to provide data again on next day.

After analysis two possible root causes were identified: (1) an intermittent hardware failure of the Field Programmable Gate Array (FPGA) inside the Mission Interface Unit (MIU), or (2) an intermittent short circuitry of the Pin Grid Array (PGA) matrix related to thermal effects.

Science data from FM6 are looking good and are provided to users from CDAAC/TACC [36], [65].